For decades, steam has been used to decompose organic chemicals, either to produce methane or to produce hydrogen and carbon monoxide and carbon dioxide as feed to other chemical processes. Because the basic process of steam reforming of organics is endothermic, much of the development in this art has focused on how best to meet the energy requirements. Typically, if external heat was not supplied, oxygen was added to the feedstock and thereby supply heat from exothermic oxidation. The apparatus for decomposing the waste also made use of the heat inherent in the effluents via heat exchange to preheat feedstock.
Other developments in steam reforming focused on fluidized bed reactors and catalysts for achieving greater efficiencies, especially in the production of synthetic gas as fuel.
The nuclear industry annually produces a significant amount of waste which is classified as radioactively contaminated ion exchange media, sludges and solvents. This waste is managed in various ways before being disposed of in bedrock chambers or by shallow land burial. Management of radioactive wastes is technically complex and, as a rule, leads to increased volumes that in turn increase storage costs. A process that results in reducing the volume and chemical reactivity of the waste disposed of is therefore highly desirable.
Ion exchange media is an organic material. The media base is usually a styrene polymer to which are grafted sulfonic acid and amine groups. The material is therefore burnable, but, when air is supplied during combustion, sulfur and nitrogen oxides are formed that in turn must be separated in some manner. Additionally, during combustion, the temperature becomes sufficiently high for radioactive cesium to be partially vaporized. The radioactivity of the burning resins could also accompany the resulting fly ash. This effect necessitates a very high performance filtration system. Accordingly, both technical and economic problems are typically associated with combustion of ion exchange media.
An alternative technique is pyrolysis. However, previously known pyrolysis methods in this field are deficient in several aspects and, in particular, no one has succeeded in devising a pyrolysis process that provides a comprehensive solution to the problem of sulfur and nitrogen-containing radioactive waste, and to do so under acceptable economic stipulations. See for example U.S. Pat. Nos. 5,424,042, 5,470,738, 5,427,738, 4,628,837, 4,636,335, and 4,654,172, and Swedish Patent SE-B 8405113-5.
Ion exchange media are not the only types of organic wastes generated by the nuclear industry, nor are they the only types of radioactive wastes generated by other industries. Some industries generate mixed wastes that include both radioactive waste and chemical wastes. The chemical wastes, for example, can include organic solvents such as trichloroethylene or PCBs. Mixed wastes are especially difficult to deal with because different and sometimes conflicting regulations apply to their dual hazards.
There is a need for a process that can efficiently decompose wastes containing radioactive contaminants and to do so in a way that reduces the volume and chemical reactivity of the waste residue remaining after decomposition.